Recently, along development of compact information equipment, electronic components to be mounted thereon have been downsized rapidly. A ball grid alley (hereinafter referred to as “BGA”) having electrodes at its rear surface is applied to such electronic components in order to cope with a narrowed connection terminal and a reduced mounting area because of the downsizing requirement.
As the electronic components to which the BGA is applied, for example, a semiconductor package is exemplified. In the semiconductor package, semiconductor chips having electrodes are sealed with resin. Solder bump is formed on each of the electrodes of the semiconductor chips. This solder bump is formed by joining a solder ball to an electrode of the semiconductor chip. The semiconductor package to which BGA is applied is placed on a printed circuit board so that each solder bump can contact an electrically conductive land of the printed circuit board, and mounted on the printed circuit board by joining the solder bump melted by heating and the land.
Now, it is required that forming a metal oxide film on a surface of a solder ball should be inhibited, in order to join the solder ball to the electrode. In addition, in a case where a solder ball is mixed with flux and then is used as solder paste, it is required that a rise in viscosity should be inhibited during storage.
Here, there is a proportional relationship between a film thickness of the oxide film formed on the surface of the solder ball and the yellow chromaticity. Accordingly, a technology has been proposed, wherein an oxide film is destroyed by the heating, with the use of a solder ball whose oxide film is equal to or less than a predetermined value, that is the yellow chromaticity is equal to or less than a predetermined value, to be able to perform joining (See Patent Document 1, for example.) This Patent Document 1 discloses that a solder ball, yellowness on the surface of which is equal to or less than 10 after being manufactured, are firstly selected, and then the storage status is strictly managed, and therefore the yellowness on the surface of the solder ball is prevented. And namely, the growth of a SnO oxide film on the surface of the solder ball is inhibited, as well as a SnO oxide film and a SnO2 oxide film are formed on the surface of the solder bump which is formed by using the solder ball.
Additionally, a technology has been proposed, wherein a rise in viscosity can be inhibited by forming an oxide film having a predetermined value on a surface of a solder ball (See Patent Document 2, for example). This Patent Document 2 discloses that an oxide film composed of tin oxide, main component of which is either SnO or SnO2 is formed on each of the surfaces of solder particles, and a rise in viscosity over time is inhibited after solder paste is manufactured by mixing and stirring them with flux.    Patent Document 1: Japanese Patent Application Publication No. 2009-248156    Patent Document 2: Japanese Patent No. 4084657